Chemically Decomposable Epoxide Resin System

ABSTRACT

The invention relates to a chemically degradable cured epoxy resin system containing one or more epoxide resins and one or more curing agents, characterized in that the curing agent contains at least one cyanate, with which the epoxy resin reacts to form an epoxy resin polymer containing cyanurates.

TECHNICAL FIELD

The invention relates to a chemically decomposable cured epoxide resinsystem comprising one or more epoxide resins and one or more curingagents. Epoxide resins, i.e. di- or poly functional mono- or oligomerswith epoxide groups can be cured with various, typically aminic oranhydridic, curing agents to cure to duromer plastic materials. Examplesfor suitable di- or poly functional mono- or oligomers are glycidylethers and glycidyl amines.

PRIOR ART

The recycling of cured epoxide resin systems is difficult due to itscross-linked structure. This applies, in particular, to chemicalrecycling under “mild” conditions. Considering the three-dimensionalnetwork such plastic materials cannot be melted or dissolved. Therefore,recycling by melting or dissolving analoguous to thermoplastics is notpossible.

There are special epoxide resin systems which are decomposable.Decomposition is possible only with compositions where compromisesregarding properties of the material are made, in particular a low glasstransition temperature T_(g). A known method is described, for example,in WO 2012/071896. One of the resin systems described therein is sold byConnoratech under the trademark “Recyclamine”.

From publications W. Dang, M. Kubouchi, H. Sembokuya, K. Tsuda, Polymer,Vol 46. No. 6, pp. 1905-1912, February 2005 and W. Dang, M. Kubouchi, S.Yamamoto, H. Sembokuya, K. Tsuda, Polymer, Vol 43. No. 10, pp.2953-2958, May 2002 decomposing methods are known having processingdurations of up to 400 hours. They are not economic due to the longdurations. Furthermore, aggressive or corrosive chemicals are necessarywith such methods. The regeneration of the decomposition productsgenerated during the decomposition is expensive.

A method for recycling printed circuit boards with epoxide resins as amatrix resin is described in WO96/16112.

A further method for recycling duromers, preferably duromers on thebasis of epoxide resins, is known from DE 198 39 083 C2. High processingtemperatures are necessary therein.

EP 1 085 044 B1 discloses the possibility to recycle anhydridicallycured epoxide resins by aminolysis. The anhydridically cured epoxideresins have great disadvantages regarding storage stability of theuncured resin-curing agent-mixture. For this reason they cannot be usedfor the production of storable prepreg-type materials. The main portionof the prepreg-type materials must be storable, since they are notfurther processed immediately after their production.

There are polymer materials where the decomposition of the desireddecomposition results can be economically achieved. An example for sucha material is described in DE 44 32 965 C1. The polymer used there is apolycyanurate polymer and is decomposed by beginning with fine grindingand carrying out aminolysis with an agent comprising at least onereactive NH₂-group. The material may be suspended in a solvent. Theproduction and the properties of such Polycyanurate polymers wasdescribed in great detail in M. Bauer, J. Bauer: Aspects of thekinetics, modelling and simulation of network build-up during cyanateester cure in: I. Hamerton (ed.): Chemistry and Technology of CyanateEster Resins, London: Blackie Academic 1994, ISBN: 0-7514-0044-0.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide an epoxide resin system ofthe above mentioned kind which is economically decomposable withouthaving to accept the impairing of the properties of the cured epoxideresin systems.

According to the invention this object is achieved in that the curingagent comprises at least one cyanate, which reacts with the epoxideresin to become an epoxide resin polymer comprising cyanurates. Acyanate is each material with the formula R—O—C≡N, wherein R is anorganic residue, which is aromatic, partial halogenated- orperhalogenated-aliphatic. R may also comprise further functional groups,which are suitable as a curing group for epoxide resins. Such a furtherfunctional group is, for example, an OH-group or a further cyanategroup. Upon polymerisation cyanurates are generated bycyclotrimerisation of the cyanate groups the cyanurates beingcharacterized by the following structure element:

The reactions of epoxides with polycyanurates and the cross-linkingstructures resulting therefrom were described in great detail in M.Bauer, J. Bauer: Aspects of the kinetics, modelling and simulation ofnetwork build-up during cyanate ester cure in: I. Hamerton (ed.):Chemistry and Technology of Cyanate Ester Resins, London: BlackieAcademic 1994, ISBN: 0-7514-0044-0.

Surprisingly, it was found that the addition of cyanates to the epoxideresin as a curing agent will lead to a very well decomposable materialwith excellent material properties. The addition of further curingagents is possible, but not necessary. The addition enables thegenerated epoxide resin polymer to be decomposed by the method describedin DE 44 32 965 C1 and to recycle it economically. The cured epoxideresin product is finely ground after reaching the end of its life time.The powder or the mixture of epoxide resin polymer and solvent is thenexposed to an aminolysis by adding an agent with at least one reactiveNH₂-group.

Alternatively, larger components with or without enforcing fibers can bedecomposed even without any shredding. In addition to requiring lessefforts this has the advantage that the enforcing fibers can be recycledwith greater length. The longer a fiber is the better its enforcingproperties will be and the higher is achievable price.

Suitable solvents are cyclic ethers, such as THF, chlorinatedcarbohydrates, such as methylene chloride, or nitrogen containingsolvents, such as pyrrolidone or NMP. Any amino group is suitable asreactive NH₂— or amino group which is able to initiate an aminolysisreaction. They are, in particular, ammonia, hydrazine, primary aminesand primary hydrazines with aliphatic or aromatic residues, which attheir end may also be substituted. The branching or chain length doesnot have an impact as long as the amino function has a sufficientreactivity towards the cyanurate groups. With such a methoddecomposition products are obtained which are more useful than commonepoxide resin systems known in the prior art. It is, of course, alsopossible to carry out the decomposition without grinding or suspensionin a solvent. Furthermore, different decomposing methods are possiblewhich can be also applied to polycyanurate containing epoxide systems,such as alcoholysis with mono-, die- or poly functional alcohols,secondary amines and alcohols.

Examples for suitable cyanates are compounds having the formulas II toVI, where the bivalent residue connecting the cyanate groups correspondsto the residue in formula I:

V a) for R⁸═H and R⁹═CH₃ the corresponding cyanate is designated as1,1′-bis(4-cyanatophenyle)ethane, which is commercially available underthe trademark PRIMASET™ by LONZA AG.

V a) for R⁸═CH₃ and R⁹═CH₃ the corresponding cyanate is designated as2,2′-Bis(4-cyanatophenyl)propylidene, which is commercially availableunder the trademark PRIMASET™ by LONZA AG.

Wherein R³ to R⁶ are independently from each other H, C₁-C₁₀-alkyl,C₃-C₈-cycloalkyl, C₁-C₁₀-alkoxy, halogen or phenyl, wherein the alkyl-or arylgroups may be fluorinated or partially fluorinated; Z is achemical compound, SO₂, CF₂, CH₂, CH(CH₃), isopropyl,hexafluoroisopropyl, alkyl, O, NR⁷, N═N, CH═CH, CO—O, CH═N, CH═N—N═CH,alkyl-O-alkyl with C₁-C₈-alkyl, dicyclopentadienyl, S, C(CH₃)₂ orC(CH₃)₂-phenyl-C(CH₃)₂, and R⁷ is H, C₁-C₁₀-alkyl, preferablyC₁-C₅-alkyl; R⁸ and R⁹ each are independent from each other H,unsubstituted or with —OCN substituted aryl, in particular unsubstitutedor with —OCN substituted phenyl, unsubstituted or substituted, inparticular fluorinated or partially fluorinated alkyl, preferablyC₁-C₅-alkyl and particularly preferred CH₃ or F₃; R¹⁰ may beunsubstituted or with OCN substituted aryl, in particular unsubstitutedor with —OCN substituted phenyl, unsubstituted or substituted, inparticular fluorinated or partially fluorinated alkyl, preferablyC₁-C₅-alkyl and particularly preferred may be CH₃ or CF₃ and n is 0 to20.

A further group can be, for example, obtained from compounds having theformula VII to IX:

wherein R¹¹ to R¹³ are alkylen-groups with 1, 2 or more carbon atoms,which are partially or entirely fluorinated. Examples are —CH₃,—CH₂—CH₃, —CH═CH₂, —CHF—CF₃ or —C(R′)₂—R″—C(R′)₃, wherein the residuesR′ may be the same or different and may be a hydrogen- or fluorine atomand be a further optionally substituted and preferably fluorinatedalkyl- or alkenyl group with preferably 1 to 6, more preferably 2 to 4carbon atoms and the residue R″ is a non-aromatic carbohydrate groupcontaining at least one double bond, preferably an alkylene group with 2to 12 carbon atoms. The indices m, n and o in formula VIII areindependent from each other and are preferably between 0 and 12; wile informula IX Z may be, for example, a chemical group which is selectedfrom SO₂, CF₂, CH₂, CHF, CH(CH₃)₂, isopropylene, hexafluoroisopropylene,fluorinated or partially fluorinated n- or iso-C₁-C₁₈-alkylene groups,O, NR¹⁴, N═N, CH═CH, —(C═O)—O—, CH═N, —C═C—, alkyl-O-alkyl with 1 to 18carbon atoms which are optionally partial or entirely fluorinated, S,Si(CH₃)₂, Si(CH)₂[O—Si(CH₃)₂]_(p) wherein p is between 1 and 12, or

wherein R¹⁴ is a hydrogen or a C₁-C₁₈ alkyl.

It is understood, that different cyanates with similar properties mayalso be used which react to be polycyanurates.

Preferably, the cyanate content is at least 30% of the functional groupsin the copolymer, preferably 50% of the functional groups in thecopolymer and most preferably 70% of the functional groups in thecopolymer. A higher portion of the polycyanurates simplifies thedecomposition in the above described method. It has been shown, however,that there is a relatively sharp limit for the portion where thedecomposition is still economic and can be chemically well operated.Such limit depends, amongst others, from the selection of the usedmaterials in order to ensure the required usage properties of the curedresin which shall be recycled.

The epoxide resins are formed preferably from aromatic epoxide resins.The aromatic epoxide resins are formed either of phenols, such asglycidether or of amines, such as, for example, glycidamines. It is,however, also possible to use aliphatic epoxide resins or mixturesthereof.

In a preferred embodiment of the invention it is provided that aromaticcyanates are used. Compared to partially halogenated or perhalogenatedaliphatic cyanates they are well available and less expensive.

In addition to epoxide resin and cyanates further additives can becomprised in the system. Further additives may be, for example, mono-,die- and higher functional phenols or impact modificators, the latteralso being called tougheners. An example for a difunctional phenol isBisphenol A. The property profile of the resulting polymer can beadapted by such additives.

In a further modification of the invention Bisphenol A is provided as anadditional copolymerizing component. The further component can beprovided with at least 5 mass-%, preferably with at least 10 mass-%.

Fiber enforced plastics, mainly cross-linked plastics are used in agreat number of applications. Examples are aeroplanes, rail vehicleconstruction, wind energy plants and in the most recent past automotiveindustry.

The epoxide resin systems according to the present invention may also beused as matrix resins for fiber reinforced plastics. Glass fibers andcarbon fibers may be considered as reinforcing fibers. Obviously,different reinforcing fibers may also be used.

If cross-linked polymers are used as a matrix according to the prior artit is difficult to recycle the fiber-reinforced composites. Thereinforcing fibers are much damaged by pyrolysis or mechanicalprocesses.

The use of the epoxide resin systems according to the present inventionas a matrix polymer provides the possibility to remove the resin matrixwithout considerable impact on the mechanical properties of thereinforcing fibers. The reinforcing fibers are not reduced in length bythe described recycling method or only very little. This is advantageousfor their further use as reinforcing fibers. The longer the reinforcingfibers are, the better are their mechanical properties with respect tothe reinforcement and the higher is the achievable sales value thereof.

Modifications of the invention are subject matter of the subclaims. Anembodiment is described below in greater detail with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the dependency of the sample mass on the decompositionduration for different samples at 20° C.

FIG. 2 shows the dependency of the sample mass on the decompositionduration for different samples at 40° C.

FIG. 3 shows the duration until complete decomposition for differentsamples in refluxing 2-aminoethanol.

FIG. 4 shows the glass transition temperatures depending on thecomposition, the mass content of DGEBF, ω_(DGEBF) in %.

DESCRIPTION OF THE EMBODIMENT

FIG. 1 shows the dependency of normed sample mass on the ordinate axison the decomposition duration on the abscissa axis for different samplesat 20° C. 2-aminoethanol was used as recycling agent. The homopolymer1,1′-bis(4-cyanatophenyl)ethane, also called LECY, shown in formula Vcan be decomposed in a relatively short period of time. This isillustrated by the graph 97 in FIG. 1. The decomposition times increasewith increasing DGEBF-content when LECY is used as a curing agent forthe epoxide resin diglycidyl ether of Bisphenol F, also called DGEBF.The polymer 80 with 40 mass-% DGEBF shows a different decompositionbehaviour than those with less or more epoxide resin: the sample bodydecomposed into several small pieces which prohibited an exact weighing.The polymer 77 has a DGEBF mass content of 10 mass-%, the polymer 78 amass content of 20 mass-%, the polymer 79 a mass content of 30 mass-%and the polymer 81 a mass content of 50 mass-%. No significantdecomposing could be found under such conditions during the experimentduration for polymer 81.

FIG. 2 shows the dependency of the sample mass on the decompositionduration up to the complete decomposition of polymers 97 and 77 at 40°C. The sample masses, normed to 100%, are represented by the ordinatewhile the corresponding decomposition durations are indicated on theabscissa in hours. 2-aminoethanol was used as a recycling agent. It canbe recognized that the decomposition duration strongly increases if thepolycyanurate content is below a threshold. The polymer 80 with 40mass-% DGEBF shows a different decomposition behaviour than those withless epoxide resin: the sample body decomposed to a plurality of smallpieces which prohibited exact weighing. Due to this circumstances onlyindividual measuring values are present in FIG. 2 for the polymer 80.The polymer 77 has a DGEBF mass content of 10-%, the polymer 78 has amass content of 20 mass-%, the polymer 79 has a mass content of 30mass-%, and the polymer 81 has a mass content of 50 mass-%. Even with areaction at 40° C. a sufficient decomposition could not be shown for thepolymer 81.

FIG. 3 shows the decomposition time depending on the composition at theboiling point of the used recycling agent 2-aminoethanol. The bar 517indicates the decomposition duration of a LECY cured DGEBF polymer withadditional coreactants Bisphenol A, this polymer mixture can also bedecomposed. The bar 518 indicates LECY cured epoxide resin withtoughener CBTN X8 and the bar 519 indicates LECY cured epoxide resinwith toughener WAX, such compositions are also decomposable. Thedecomposition of the polymer 210 with 58 mass % DGEBF lasts longer than48 h. The duration until full decomposition increases with increasingepoxide content which is illustrated by the bars polymer 97 with 0mass-% DGEBF to polymer 210 with 58 mass-%. The polymer 77 has aDGEBF-mass content of 10 mass-%, the polymer 78 has a mass content of 20mass-%, the polymer 79 has a mass content of 30 mass-%, the polymer 80has a mass content of 40 mass-% and the polymer 81 has a mass content of50 mass-%. The polymers 207 to 210 have mass contents from 52 mass-% to58 mass-%, increasing in steps of 2 mass-%.

FIG. 4 shows the glass transition temperature depending on the masscontent of epoxide resin EPIKOTE™ Resin, which is also called DGEBF. Itcan be seen also, that a strong decrease of the glass transitiontemperature occurs from an epoxide resin content of 60 mass-% on.

For investigating the recycling capability of epoxide resin polymerscured with cyanates pure resin plates were produced with variouscompositions. Commercially from Lonza available Primaset™ LECY was usedas cyanate. Bisphenol-F-diglycidylether based epoxide resin EPIKOTE™Resin 862 by Momentive was used as epoxide resin. Technical Bisphenol Awas used as co-component.

Alumina moulds served to cast the pure resin plates. LECY was heated toabout 40-50° C. in order to lower the viscosity and thereby ensure exactweighing. LECY was propounded in a beaker and the epoxide resin added.The mixture was homogenized by means of a magnetic stir bar or KPGstirrer. If additional impact modificator or Bisphenol A was added tothe mixture a KPG stirrer was used at all times. Bisphenol A provided inthe form of small balls was pulverized by means of a ball mill and addedin small portions. The homogenized mixture was then poured into thealumina mould for curing and heated in a circulating air drying cabinet.A pure resin plate was made of pure LECY for reference.

The respective curing programs depending on the composition of thesampling plate are indicated in the following table:

formula EPIKOTE 862 + EPIKOTE 862 + LECY LECY + LECY (+Toughener)Bisphenol A curing 1. 120° C. 1. 120° C. 1. 120° C. programme 2. 2 h180° C. 2. 0.5 h 150° C. 2. 0.5 h 150° C. heating heating heating 3. 30h 180° C. 3. 4 h 150° C. 3. 4 h 150° C. 4. 0.5 h 250° C. 4. 0.5 h 170°C. 4. 0.5 h 170° C. heating heating heating 5. 4 h 250° C. 5. 3 h 170°C. 5. 3 h 170° C. 6. 1 h 220° C. heating 7. 1 h 220° C.

Sampling plates were made for recycling experiments having a compositionas indicated in the following table:

plate ω_(L10) ω_(DGEBF) ω_(Bisphenol A) ω_(CBTNX8) ω_(WAX) number[mass-%] [mass-%] [mass-%] [mass-%] [mass-%] 97 100 — — — — 77 90 10 — —— 78 80 20 — — — 79 70 30 — — — 80 60 40 — — — 81 50 50 — — — 82 40 60 —— — 83 30 70 — — — 207 48 52 — — — 208 46 54 — — — 209 44 56 — — — 21042 58 — — — 517 40 40 20 — — 518 45 45 — 10 — 519 45 45 — — 10

The sample plates were sawn into pieces of 20×30×6 mm³ each having amass of about 5 g. One sample of each of the polymers 77-83 wasintroduced in a duran screw cap glass with the 4-times amount ofaminolysis reagent 2-aminoethanol and the glass was closed. One sampleof each of the polymers was tempered at 20° C. and one sample of each ofthe polymers was tempered at 40° C. and the polymer samples taken afterwell-defined periods of time and weighed. This served to monitor thedecomposition of the polymers.

Furthermore, the polymers 77-83, 207-210 and 517-519 were introduced ina 50 ml conical flask with Dimroth condenser and also the 4-times amountof 2-aminoethanol was added. With such experiments the 2-aminoethanolwas heated in an oil bath to 172° C. whereby the experiments werecarried out under reflux conditions. The time required for the fulldecomposition was recorded as the decomposition duration of the polymersample.

The glass transition temperature (tan δ) was determined by means oftorsion-dynamic mechanical analysis (DMA).

1. Cured epoxide resin system which is chemically decomposable byaminolysis with reactive NH₂— or other amino groups and/or alcoholysiswith mono-, di-, or poly functional alcohols comprising one or moreepoxide resins and one or more curing agents wherein the curing agentcomprises at least one cyanate with the formula R—O—C═N, wherein R is anorganic residue which is aromatic, partially halogenated- orperhalogenated-aliphatic, which reacts with the epoxide resin to becomean epoxide resin polymer comprising cyanurates and wherein the contentof cyanate groups before curing is at least 30% of the functional groupsof the monomers which form the copolymer.
 2. Epoxide resin systemaccording to claim 1, characterized in that the content of cyanategroups before curing is at least 50% of the functional groups in thecopolymer and preferably at least 70% of the functional groups of themonomers which form the copolymer.
 3. Epoxide resin system according toclaim 1, characterized in that the epoxide resins are aromatic epoxideresins.
 4. Epoxide resin system according to claim 1, characterized inthat the cyanates are aromatic cyanate resins.
 5. Epoxide resin systemaccording to claim 1, characterized in that further additives arecomprised and the cyanate content is adapted to the content of additivesunder consideration of the cross-linking density.
 6. Epoxide resinsystem according to claim 1, characterized in that Bisphenol A isprovided as an additional curing agent.
 7. Epoxide resin systemaccording to claim 6, characterized in that Bisphenol A is provided withat least 5 mass-%, preferably with at least 10 mass-%.
 8. Epoxide resinsystem according to claim 1, characterized by reinforcing fibers in amatrix of epoxide resin.
 9. Epoxide resin system according to claim 8,characterized in that the reinforcing fibers are glass fibers and/orcarbon fibers.
 10. Use of an epoxide resin system according to claim 1as fiber reinforced plastics in aeroplanes, in rail vehicleconstruction, in wind energy plants and in the automotive industry.